Recently, so-called renewable energy, for example, solar power generation, wind power generation and wave power generation, has attracted attention as new energy sources in place of fossil fuel, such as petroleum, and nuclear power. However, the renewable energy is strongly affected by weather and, therefore, is very unstable in output. When supplying a large amount of the renewable energy to power grids, it is necessary, for example, to provide high-capacity batteries to level the fluctuating output.
One example of the high-capacity battery is a redox flow battery. The redox flow battery comprises two kinds of ionic solutions which are separated with a cation exchange membrane, in each of which solutions an electrode is placed. At the electrodes, an oxidation reaction and a reduction reaction occur simultaneously to cause charge and discharge. For example, in a redox flow battery in which an aqueous sulfuric acid solution containing vanadium is used at each pole, vanadium (IV) is oxidized to vanadium (V) at the positive pole, and vanadium (III) is reduced to vanadium (II) at the negative pole for the charge. In the discharge, the reverse reactions occur. The redox flow batteries have an advantage that it is easy to provide large-capacity ones. The batteries function at room temperature. Further, no combustible or explosive material is used or generate in the batteries. Therefore, they are very safe, compared to sodium-sulfur batteries and lithium ion secondary batteries.
Electrodes of redox flow batteries are immersed in an electrolyte such as an aqueous sulfuric acid solution. Further, oxidation and reduction reactions occur at the electrodes. Therefore, the electrodes need to have high electric conductivity and chemical resistance. Accordingly, carbon fiber assemblies or platinum plating have been used in the electrodes. However, the carbon fiber assemblies allow a liquid to pass through and, therefore, have a problem that the junction with a copper wire is affected by an aqueous sulfuric acid solution passed through the assemblies. The platinum plating is a very good conductor and is excellent in chemical resistance, but has a drawback that it is of a noble metal and costly.
Therefore, electrically conductive resin films comprising electrically conductive carbon such as ketjen black have been proposed as the electrodes (see Patent Literatures 1 to 4), or electrodes made of carbon fiber assemblies or of copper plates have been coated with the aforesaid electrically conductive resin film. However, when the electrically conductive resin film comprises an enough amount of electrically conductive carbon to have sufficiently high electric conductivity, the film is very poor in tensile elongation, durability to bending or flexibility and, therefore, easily breaks by a physical force. When the amount of electrically conductive carbon is so small to secure good tensile elongation, durability to bending and flexibility, the volume resistivity of the film exceeds 10 Ω·cm to increase internal resistance of a redox flow battery in which the film is used as the electrodes or coatings on the electrodes, which makes the battery unsatisfactory.
In recent years, carbon nanotubes have attracted attention as conductive carbon and been expected to solve the aforesaid problems (see Patent Literature 5 and Non-Patent Literature 1). However, carbon nanotubes have problems that they are difficult to be spread and, therefore, very difficult to be dispersed in resins. Therefore, a large amount of carbon nanotubes must be added, like ketjen black, in order to obtain satisfactorily high electric conductivity and, therefore, the resulting electrically conductive resin film is practically inferior in tensile elongation, durability to bending and flexibility. When high shearing stress is applied in spreading and dispersion steps in order to obtain better spreading and dispersion of carbon nanotubes, the carbon nanotubes break and, therefore, it is again necessary to use a large amount of carbon nanotubes in order to obtain satisfactorily high electric conductivity.
An electrically conductive film has been proposed, made of a composition obtained by mixing carbon black or carbon nanotubes with propylene-olefin copolymer wax to prepare a master batch which is then mixed with an organic polymer (see Patent Literatures 6 and 7). The master batch makes it possible to incorporate carbon black or carbon nanotubes in a large amount into the polymer. However, the electric conductivity of the resulting film is not enough.